The present invention relates to fault locating systems and methods for four-wire communication networks having at least one supervisory station and a plurality of unmanned repeater stations. In known apparatus of this type, test signals generally in the form of pulses are sent from the supervisory station through a pair of wires to all of the repeater stations. In each of the repeater stations an electrical network couples test signals from the first pair of wires to the second pair of wires so that said test signals received at a repeater station are transmitted back to the supervisory station. On the basis of missing return signals it is then possible to locate the line segment or the repeater station which is inoperative. Such a method and arrangement is described in U.S. Pat. No. 3,312,791. In this known method and arrangement a pulse-amplitude modulated frequency outside of the communication frequencies is utilized as test signal. At each individual repeater station, the network connecting the output of the amplifier connected to the first pair of wires to the input of the amplifier connected to the second pair of wires is a filter which blocks the communication signals and passes only signals of the frequency associated with the test signals. The same filters are used in all of the repeater stations and are therefore readily interchangeable. Because of the difference in distance between individual repeater stations and the supervisory station, the returning test signals looped via each repeater station arrive at different times back at the supervisory station, so that actually a pulse sequence is received at the supervisory station in response to each pulse sent out as a test signal. This pulse sequence is displayed on an oscillograph so that the number of returned pulses can be counted. Thus for example if one supervisory station supervises m repeater stations and the number of returned pulses is only equal to q where q is less than m, then the fault exists at the (q+1)th repeater station as counted from the supervisory station.
This system and method works well when a relatively small number of repeater stations is supervised by one supervisory station. Under these conditions the number of returned signals can readily be counted on the screen of the oscillograph. However when one supervisory station supervises a very large number of repeater stations, such a display becomes very difficult to read. Thus for example for a single supervisory station which supervises more than forty repeater stations as is often the case for carrier frequency systems having a large number of channels and a small distance between repeater stations, the display becomes almost impossible to read since the pulses are spaced so closely to each other that it is difficult or impossible to arrive at the correct count.